After a fibrous web, such as paper, is formed on a papermaking machine, the web is typically advanced through a calender which improves various parameters of the web. For example, surface smoothness and caliper are parameters of the web and resultant sheet which can be controlled by advancing the web through a calender.
One particularly advantageous type of calender is known as a wet stack calender. In a wet stack calender, several rolls are arranged in nipping contact and with their axes generally vertically aligned one above the other. The web is wound down through the stack of rolls in a serpentine fashion alternating from one side of one roll, through a nip, and around the other side of the subjacent roll until the web exits from the bottom of the stack. The lowermost roll is sometimes referred to as the king roll. One or more waterboxes are arranged adjacent to the side of a roll which is exposed and not in contact with the web. The waterboxes are usually positioned at intermediate positions in the stack (e.g., adjacent to the second or third rolls up from the king roll in a five-roll stack).
As the roll rotates into the nip, the inrunning surface portion of the roll is supplied with water from the waterbox which carries the water into the nip such that the water is transferred to the web. One example of such a wet stack calender is illustrated in U.S. Pat. No. 5,607,553 which illustrates the use of a brush spray device for supplying water to the exposed inrunning surface portion of a roll. Similarly, a waterbox having a metering element such as a blade for applying a film to the surface of a roll is disclosed in U.S. Pat. No. 5,522,312.
One recurring problem faced by paper manufacturers is ensuring uniformity of web properties in the cross-machine direction. In other words, in order to provide a more uniform and therefore superior product, it is desirable to reduce variation in the parameters across the width of the web. These variations can be caused by many factors including, for example, the effects of gravity on the various components of the papermaking machine.
One approach to adjusting the profile of the web has been to vary the diameter of the king roll in the cross-machine direction. For example, the king roll can be provided with a machined crown to account for the effects of gravity on the roll. The crown may also be controlled during machine operation by providing a roll having an outer shell and an interior hydraulic chamber provided in sealing contact with the inner surface of the outer shell. Accordingly, to control the crown of the king roll, the hydraulic pressure inside the chamber is adjusted which varies the deflection of the outer shell. The king roll may also be provided with one or more hydraulically mounted shoes which press against the inner surface of the outer shell to radially expand the roll. The actuation of the shoes is sometimes independently controlled so that the pressure in the nip can be adjusted to a desired profile.
U.S. Pat. No. 4,114,528 discloses a device for controlling the caliper of a web issuing from a nip of a plural roll device, such as a calender. One of the rolls of the device is subjected to a multiplicity of air jets which impinge air of various temperatures on the roll. By increasing the temperature of the air in any one or group of the jets, the material of the roll is slightly expanded which increases the diameter of the roll adjacent to that air jet and increases the pressure in the nip, thereby reducing the caliper of the web.
Prior attempts at profile control have been limited to smoothness and caliper control in the nip with the king roll. In other words, it has been considered the best approach to vary the pressure in the nip formed with the king roll (the last nip to which the web is subjected in the stack) to vary the caliper and smoothness profile of the web.
While proper caliper and smoothness are important to a finished web, it has been determined that the moisture profile of the web before final drying is also important to proper web formation. The water applied to the web in the calender helps add surface smoothness without a substantial decrease in bulk. However, if the moisture profile in the cross-machine direction is not sufficiently uniform, various other properties of the web can be deleteriously affected. In addition, the runnability of the machine can be impaired. With the conventional approach of varying the nip pressure of the final nip with the king roll, the properties which are most affected are caliper and smoothness profile. Although the moisture profile can be varied somewhat by adjusting this nip pressure, the resultant changes in caliper and smoothness prevent significant control of the moisture profile and the advantages attendant to a proper profile.
Accordingly, there has become a great desire in the papermaking industry to provide control of the moisture profile of the web in a wet calender stack. Separating the control of the moisture profile from the control of the web caliper and smoothness has not previously been feasible. There is a significant need for a calender wherein the moisture profile of the web can be isolated and separately controlled.